1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for adjusting DC feed control to a subscriber loop.
2. Description of the Related Art
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The primary functions of the line card may include supplying talk battery, performing impedance matching, determining whether a telephonic equipment is on-hook or off-hook, and handling ringing signals, voice signals, and testing signals.
Subscriber line interface circuits generally include a switch-hook detection circuit that supervises telephone operation by detecting whether a telephone is either on-hook or off-hook. An xe2x80x9coff-hookxe2x80x9d condition occurs when an end user lifts the handset of a telephone from the cradle, thereby activating the telephone""s hook switch. Conversely, an xe2x80x9con-hookxe2x80x9d condition occurs when the handset is placed back in the telephone cradle, thereby terminating the telephone service. Upon detecting the on-hook or off-hook condition, the switch-hook information is passed to the system software of the line card, which then either provides or terminates service.
Upon requesting service, a user may establish a connection with a remote user by dialing the telephone number of that user. Telephone dialing may be in the form of dial pulses or tones. Pulse dialing includes generating of a series of electrical interrupts or pulses across the telephone line. One method of generating electrical pulses is by toggling between on-hook and off-hook states such that each transition from an on-hook to off-hook state represents one pulse. The number of pulses generated usually represent the digit that is dialed.
The off-hook/on-hook ratio (also referred to as make/break ratio) applied to the loop during pulse dialing is typically 40 to 60; that is, the loop is closed 40 percent of the time and is open 60 percent of the time. The break interval is generally allowed to vary from about 58 percent to 64 percent. However, because of the pulse distortion caused by the loop, the pulse receivers or detectors in the central office must be able to properly respond to a break interval of 55 percent to 65 percent.
A telephone system needs DC feed to control the battery feed to the subscriber loop. DC feed delivers enough power for long loop and gradually reduces the power for short loop. A digital signal processing algorithm controls the DC feed curve. During the transition from off-hook to on-hook, the loop impedance changes from low to high and the measured loop voltage between the tip and ring terminals goes from low to high. Generally, the DC feed control is slow to react to sudden changes in the loop impedance. As such, if the loop impedance is high enough during an off-hook to on-hook transition, the DC feed driver may be saturated. During saturation, the measured loop voltage between the tip and ring terminals and the flowing current may not accurately reflect the loop impedance, ZLOOP, a parameter which is generally used for switch hook detection. Thus, an inaccurate measurement of the loop resistance may result in a false detection, thereby causing switch-hook distortion. Additionally, switch-hook distortion may also occur during pulse dialing because of capacitance and inductance present on the subscriber line.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a method is provided for adjusting DC feed control of a connection. The method includes receiving a signal from the connection and determining if the change in the signal is greater than a first preselected value. The method further includes adjusting the DC feed in response to determining that the change in the signal is greater than the first preselected value.
In another aspect of the present invention, an apparatus is provided for adjusting DC feed control of a connection. The apparatus includes means for receiving a signal from the connection and comparator logic adapted to determine if the change in the signal is greater than a first preselected value. The apparatus further includes means for adjusting the DC feed in response to determining that the change in the signal is greater than the first preselected value.